Various compositions have been proposed for stripping metal coatings from workpieces of other metal onto which the coatings have been deposited by electroplating or chemical plating techniques. Frequently, it is found that there is an imperfection in the plated deposit, such as discoloration or poor adhesion, or an imperfection of the workpiece itself, which will require the removal of the plated deposit for reworking of the part or for recovery of the metal of the deposit. With the soaring cost of precious metals, it has become extremely important that plated deposits of such metals be removed completely and with minimum contamination so that the precious metal values may be recovered from the stripping solution conveniently and economically. Accordingly, it is extremely desirable that the stripping solution be one which will remove the precious metal or other plated metal deposit rapidly and completely without appreciably attacking the substrate.
In Grunwald U.S. Pat. No. 3,242,090, there is disclosed a gold stripping composition comprising a complex metal tetracyanide salt, a nitrosubstituted aromatic compound such as nitrobenzenesulfonic acid, nitobenzoic acid, nitrochlorobenzene, nitrophenol and nitrobenzaldehyde. The metal forming the complex of the patented composition is selected from the group consisting of zinc, cadmium and copper. Another prior art composition is disclosed in Springer et al. U.S. Pat. No. 2,649,361 and comprises sodium cyanide, sodium nitrobenzenesulfonate and sodium hydroxide. The compositions of the Grunwald patent and the Springer patent have enjoyed substantial utilization as strippers of gold from various types of substrates.
Generally, it has been found that such stripping compositions have not removed the metal deposits as rapidly as might be desirable and that there has been a tendency to attack various metal substrates. Moreover, there has been a continuing need to develop dry powder compositions permitting make-up of the aqueous stripping composition rapidly and easily with a minimum of danger to the operator. Lastly, there has been a continuing need for a composition which would operate at a high rate of speed at ambient to low elevated temperatures.
It is an object of the present invention to provide a novel dry powder composition which, upon admixture with water, will produce a highly effective solution for stripping gold and silver from various substrates.
It is also an object to provide such a composition which exhibits reasonably long shelf life and which is relatively economical.
It is a further object to provide a method for stripping gold and silver from various substrates which is conveniently and economically practiced using for initial make-up and for replenishment a dry powder composition.
Another object is to provide such a method in which gold values stripped from the substrate may be readily recovered on an economical basis.